1. Field of the Invention
The present invention relates to a lightwave circuit device, an optical multiplexer, and an optical demultiplexer.
2. Description of the Related Art
As a lightwave circuit device, a coarse wavelength division multiplexing (CWDM) filter that makes use of a planar lightwave circuit (PLC) is disclosed is Kitoh et al., Proceedings of the 2002 IEICE General Conference, C-3-125 (2002), p. 257. With this CWDM filter, a plurality of lattice filters are disposed in tandem. This CWDM filter demultiplexes light of multiplexed wavelength into four lightwaves of different wavelengths.
As another lightwave circuit device, an optical multiplexer or optical demultiplexer in which a plurality of Mach-Zehnder interferometers are connected in a multistage tree is disclosed in Japanese Patent Application No. 2003-149472. Each of the Mach-Zehnder interferometers includes a first optical coupler, two optical waveguides connected to the first optical coupler, and a second optical coupler connected to the two optical waveguides. One of the two optical waveguides functions as a delay circuit.
The wavelengths of lightwaves multiplexed or demultiplexed by one of the above-mentioned lightwave circuit devices are predetermined for each of the lightwave circuit devices. If lightwaves whose wavelengths differ from the predetermined wavelengths are inputted to the above-mentioned lightwave circuit device, such lightwaves cannot be multiplexed. Similarly, if light in which lightwaves whose wavelengths differ from the predetermined wavelengths are multiplexed is inputted to the above-mentioned lightwave circuit device, it is impossible to demultiplex the light into individual lightwaves.